The present invention relates to a dielectric composition based on polyarylalkanes which has improved dielectric properties and to its use in electrical equipment operating at high temperature, especially distribution transformers.
A trend nowadays is to employ distribution transformers in respect of which attempts are made to increase the power while maintaining the same dimensions.
This increase in power per unit volume results in an increase in the operating temperatures, which may run from 60.degree. C. -80.degree. C. in the case of conventional transformers, to 150.degree. C. or 200.degree. C. in the case of so-called "high temperature" distribution transformers.
For this new type of equipment, liquids are being sought which have good dielectric properties even at high temperature, a good heat stability, as well as low vapor pressure and a viscosity that is sufficiently low for removing the thermal energy.
The insulating oils commonly employed, such as mineral oils, silicone oils and pentaerythritol esters have a relatively mediocre high-temperature behaviour. In the case of silicone oils in particular, a lowering of the breakdown voltage was observed when the temperature rose (study performed by General Electric Co. for the U.S. Department of Energy--report HCP/T-2115 published in February 1979--FIG. 3.3, page 35) .
This behaviour appears to be general, because it has also been observed in synthetic liquids such as hexane (IEEE Trans. Electr. Insul. vol. E1-13, No. 4, August 1978, p. 263) and in mineral oils ("Insulating Materials for Design and Practice" by Franck M. Clark, page 151--Publ. John Wiley and Soils, Inc.).
This introduces in particular the disadvantage of not being able to keep the same distances between the conductors if it is desired to increase the power of the equipment and, consequently, results in an increase in its size.
Furthermore, mineral oils and silicone oils exhibit limited heat stabilities.
In addition, silicone oils are poor heat transfer agents.